87 3 Ionothermal Synthesis of Zeolites and Other Porous Materials Russell E. Morris 3.1 Introduction Innovation in zeolite synthesis remains an important aspect in the search for new framework materials with potential applications. The driver for the search for new zeolites and related solids is not only the need to provide materials for new and emerging applications [1] but is also the desire to understand how these fascinating materials are made, and ultimately how to control their architectures. Given that the applications of zeolites (and other porous solids such as metal organic frameworks) are intimately connected with their architecture, new synthetic methods that aim to understand how their structure can be controlled are very important. The core strategy that has been exercised over recent years has been the development of new organic compounds that can be used as structure directing agents (SDAs or templates). Simply preparing new SDAs has led to a significant increase in the numbers of zeolite structures over recent years. This is still a method that produces some remarkable new materials, such as IM-12 [2]. However, other more innovative methods have also made their impact, both in terms of finding new ways to recycle templates [3] and in completely new synthesis concepts such as the use of fluoride mineralizers and charge density mismatch solutions [4–7]. The use of fluoride as a mineralizing agent to improve the solubility of the starting reagents and to catalyze the formation of bonds in the target frameworks has been exploited by several groups to produce several new materials over recent years [4–6]. Charge density mismatch solutions, developed by workers at UOP, have also provided routes to new solids [7]. In this process, stable solutions of the inorganic starting materials are prepared by using organic cations that do not make good SDAs because their charge density does not match that of the chemical composition of the inorganic framework which will be formed. Crystallization of the framework is then initiated by addition of another SDA, often in quite small amounts. High-throughput methods have also been applied with some distinct success, particularly by Corma’s group in Valencia [8]. In our laboratory, we have pioneered the use of ionic liquids (ILs) as both the solvent and SDA simultaneously [9]. The change from a molecular solvent, such as water or organic molecules, to Zeolites and Catalysis, Synthesis, Reactions and Applications. Vol. 1. Edited by Jiˇ r´ ı ˇ Cejka, Avelino Corma, and Stacey Zones Copyright 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-32514-6
Transcript
87
3Ionothermal Synthesis of Zeolites and Other Porous MaterialsRussell E. Morris
3.1Introduction
Innovation in zeolite synthesis remains an important aspect in the search for newframework materials with potential applications. The driver for the search for newzeolites and related solids is not only the need to provide materials for new andemerging applications [1] but is also the desire to understand how these fascinatingmaterials are made, and ultimately how to control their architectures. Given that theapplications of zeolites (and other porous solids such as metal organic frameworks)are intimately connected with their architecture, new synthetic methods that aimto understand how their structure can be controlled are very important.
The core strategy that has been exercised over recent years has been thedevelopment of new organic compounds that can be used as structure directingagents (SDAs or templates). Simply preparing new SDAs has led to a significantincrease in the numbers of zeolite structures over recent years. This is still a methodthat produces some remarkable new materials, such as IM-12 [2]. However, othermore innovative methods have also made their impact, both in terms of findingnew ways to recycle templates [3] and in completely new synthesis concepts suchas the use of fluoride mineralizers and charge density mismatch solutions [4–7].The use of fluoride as a mineralizing agent to improve the solubility of the startingreagents and to catalyze the formation of bonds in the target frameworks hasbeen exploited by several groups to produce several new materials over recentyears [4–6]. Charge density mismatch solutions, developed by workers at UOP,have also provided routes to new solids [7]. In this process, stable solutions ofthe inorganic starting materials are prepared by using organic cations that do notmake good SDAs because their charge density does not match that of the chemicalcomposition of the inorganic framework which will be formed. Crystallization ofthe framework is then initiated by addition of another SDA, often in quite smallamounts. High-throughput methods have also been applied with some distinctsuccess, particularly by Corma’s group in Valencia [8]. In our laboratory, we havepioneered the use of ionic liquids (ILs) as both the solvent and SDA simultaneously[9]. The change from a molecular solvent, such as water or organic molecules, to
Zeolites and Catalysis, Synthesis, Reactions and Applications. Vol. 1.Edited by Jirı Cejka, Avelino Corma, and Stacey ZonesCopyright 2010 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimISBN: 978-3-527-32514-6
88 3 Ionothermal Synthesis of Zeolites and Other Porous Materials
an ionic one changes the chemistry of the system markedly. We have given thename ionothermal synthesis to this method to delineate it from hydrothermal orsolvothermal synthesis.
Over the last few years, ILs have received great attention in many fields [10].Most of the studies have focused on the ‘‘green’’ chemistry [11] potential of thesecompounds, with particular emphasis on the drive to replace organic solventsin homogeneous catalysis [12]. The particular property of ILs that makes themenvironmentally suitable for these purposes is their low vapor pressure [13],which has significant advantages when replacing highly volatile organic solvents.However, there are many other uses of ILs in diverse areas of technology rangingfrom electrolytes in batteries and fuel cells [14], as electrodeposition solvents [15]to the use of supported IL as catalysts [16]. In some reactions, the ILs act only asinert solvents and in others the liquid plays a more active role in the reactions thattake place.
The broadest definition of an IL is any material in the liquid state that consistspredominantly of ionic species [10]. Any ionic salt that can be made molten cantherefore be classified as an ‘‘IL,’’ always assuming that the ionic components of thesolid remain intact on melting. There are many examples in the literature of moltensalts being used as the medium in which inorganic materials have been prepared[17]. Usually, these synthetic procedures take place at highly elevated temperatures,producing dense phase solids. For example, alkali metal hydroxide molten salts canbe used as the molten phase, often contained in sealed inert (such as silver) vesselsin the synthesis of many inorganic solids. In general, such molten salt synthesismethods have been used as direct replacements for traditional solid-state synthesistechniques [17]. However, the modern definition of ILs tends to concentrate onthose compounds that are liquid at relatively low temperatures and that containorganic components [18]. Room temperature ionic liquids (RTILs) are, as thename suggests, liquid at room temperature, while near room temperature ionicliquids (nRTILs) are often defined as being liquid below a certain temperature,often 100 ◦C, although this varies depending on the application envisaged for theliquids. For ionothermal synthesis, nRTILs are often defined as being liquid belowabout 200 oC, the temperatures traditionally used in hydrothermal synthesis [9].In modern usage, the term ionic liquid is almost exclusively reserved for liquidsthat contain at least one organic ion. The organic components of ILs tend to belarge and often quite asymmetric, which contribute to their low melting points bymaking efficient packing in the solid state more difficult [10].
ILs show a range of properties that make them suitable for use as mediafor the preparation of inorganic and inorganic–organic hybrid materials. Theycan be relatively polar solvents, ensuring reasonably good solubility of inorganicprecursors [19, 20]. Many (but not all) ILs have good thermal stability, enablingthem to be used at elevated temperatures.
Deep eutectic solvents (DESs) are a related class of IL, produced as a mixtureof two or more compounds that has a lower melting point than either of itsconstituents [21]. Eutectic mixtures display unusual solvent properties that are verysimilar to those shown by the ILs. High solubility can be observed (depending on
3.2 Hydrothermal, Solvothermal, and Ionothermal Synthesis 89
the eutectic mixture used) for inorganic salts, salts that are sparingly soluble inwater, aromatic acids, amino acids, and several metal oxides [22]. Advantages ofeutectic mixtures over other ILs are their ease of preparation in pure state andtheir relative nonreactivity with water. Many are biodegradable and the toxicologyof the components maybe well characterized. Eutectic mixtures based on relativelyavailable components such as urea and choline chloride are also far cheaper thansome other ILs.
Fundamentally, there is of course no real difference between an IL and amolten salt, except perhaps that the organic nature of the components of an ILintroduces much more scope for introducing functionality into the solvents. Inthe following feature chapter, the focus is on the use of the nRTILs containingorganic components as the media for materials synthesis, consistent with modernusage of the terminology. In particular, the focus is on the synthesis of templatedcrystalline materials such as zeolites and metal organic frameworks where the ILcation acts to direct the structure of the resultant inorganic or inorganic–organichybrid material.
3.2Hydrothermal, Solvothermal, and Ionothermal Synthesis
Broadly speaking, the synthesis of crystalline solid-state materials can be split intotwo main groups; those where the synthesis reaction takes place in the solid stateand those where it takes place in solution. The solid-state method usually requiresrather high temperatures to overcome difficulties in transporting the reactants tothe sites of the reaction. The high temperatures of solid-state reactions also tendto provide routes to the thermodynamically more favored phases in the systems ofinterest. Typically, this method is used to prepare solid-state oxides.
Transport in the liquid phase is obviously much easier than in solids, andsyntheses require much lower temperatures (often less than 200 oC). The mostcommonly used of this type of preparative technique is hydrothermal synthesis,where the reaction solvent is water [23]. The most common method of accomplish-ing hydrothermal synthesis is to seal the reactants inside Teflon-lined autoclaves sothat there is also significant autogenous hydrothermal pressure produced, often upto 15 bar. The lower temperatures required for hydrothermal synthesis often lead tokinetic control of the products formed, and it is much easier to prepare metastablephases using this approach than it is using traditional solid-state approaches. Theimportant reaction and crystallization processes in hydrothermal synthesis do notnecessarily take place in solution (although, of course, they can) but can occur atthe surfaces of gels present in the mixtures.
Solvothermal synthetic methods refer to the general class of using a solvent inthe synthesis of materials [24]. Of course water is by far the most important solvent,hence the special usage of the term hydrothermal to describe its use. However, thereare many other possible solvents. Alcohols, hydrocarbons, pyridine, and manyother organic solvents have all been used with varying degrees of success [24].
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As with water, these molecular solvents produce significant autogenous pressureat elevated temperatures. The solvents used in solvothermal synthesis vary widelyin their properties, from nonpolar and hydrophobic to polar and hydrophilic.
The solvents used in hydrothermal and solvothermal synthesis differ fundamen-tally from ILs in that they are molecular in nature. The ionic nature of ILs impartsparticular properties, including low vapor pressures [25] (and very little, if any,autogenous pressure is produced at a high temperature).
3.3Ionothermal Aluminophosphate Synthesis
Many ILs used currently often have chemical structures that are very similar to thestructures of commonly used SDAs (sometimes also known as templates) in thehydrothermal synthesis of zeolites and other porous materials [26]. This realizationled to the first attempts to prepare zeotype frameworks using ILs as both thesolvent and the template provider at the same time. The potential advantage of thisapproach is that the competition between the solvent and template for interactionwith any growing solid is removed when both the solvent and the template are thesame species. In principle, this may lead to improved templating of the growingzeolite crystal structure. The first work in this area, published in 2004, used1-ethyl-3-methyl imidazolium bromide (EMIM Br) and urea/choline chloride DESsto prepare several different materials depending on the conditions [9].
Since the first breakthroughs in this area, there have been many further attemptsto prepare zeotype materials. The ionothermal synthesis of aluminophosphatezeolites has been by far the most successful. Many common ILs are suitable solventsfor the preparation of these materials, with both known [27–30] and previouslyunknown [31] structure types, as well as related low-dimensional materials [32,33] being synthesized successfully. It is interesting to note that more than simplypreparing the base aluminophosphate structure, the ionothermal method is alsosuitable for incorporating the dopant metal atoms that give the frameworks theirchemical activity. Silicon (to make so-called SAPOs) [34] and many differenttetrahedral metals (Co, Mg, etc.) can all be incorporated into the ionothermallyprepared aluminophosphate zeolites, and aspects such as their catalytic activity[35] and the use of additional templates [36] show some very promising results. Adiscussion of some of the unusual concepts seen in AlPO synthesis is discussed inthe remaining sections of this chapter.
Figure 3.1 illustrates several of the SIZ-n(ST.Andrews Ionothermal Zeolite)materials that can be prepared from one particular IL–EMIM Br. Several of thesematerials have known frameworks but several others were previously unknown.The structure of SIZ-1 consists of hexagonal prismatic units known as double sixrings joined to form layers that are linked into a three-dimensional framework byunits containing four tetrahedral centers (two phosphorus and two aluminum)known as single four rings. The formula of the material is Al8(PO4)10H3·3C6H11N2
but the Al–O–P alternation is maintained. The framework is therefore interrupted
3.3 Ionothermal Aluminophosphate Synthesis 91
N N+
Br −
SIZ-1
SIZ-3 SIZ-4
SIZ-5
SIZ-6SIZ-7 SIZ-8
SIZ-9
Figure 3.1 Representatives of the SIZ-n series of alu-minophosphate zeotype structures prepared using 1-ethyl,3-methyl imidazolium bromide ionic liquids as both the sol-vent and structure directing agent.
with some unusual intraframework hydrogen bonding. The negative charge presenton the framework (caused by the existence of terminal P–O bonds) balances thecharge on the 1-methyl-3-ethyl imidazolium templates that are present in the pores.The overall structure of SIZ-1 shows a two-dimensional channel system parallelto the a and b crystallographic axes. SIZ-3, SIZ-4, SIZ-5, SIZ-8, and SIZ-9 allhave known framework structures (AEL, chabasite (CHA), AFO, AEI, and sodalite(SOD) frameworks respectively). The structure of SIZ-7 is also a novel cobaltaluminophosphate material, given the International Zeolite Association (IZA) codeSIV. However, SIZ-7 is a novel framework structure, which joins a family of relatedzeolites that includes the PHI, GIS, and merlinoite (MER) structure types. Thisfamily can be described as consisting of the double-crankshaft chain.
In SIZ-7, these chains run parallel to the crystallographic a axis in the structureand are connected to form a one-dimensional small-pore zeolite structure withwindows into the pores delineated by rings containing eight tetrahedral atoms(known as eight-ring windows). The repeat unit in the a direction is 10.2959 (4)A and equals one repeat unit of the double-crankshaft chain. These chains arelinked via four rings in both the b and c directions to form the eight-ring windows.The relative orientation of neighboring chains means that there are two types ofeight-ring channels. The two different windows are of similar size (3.66 × 3.26 Aand 3.40 × 3.52 A) but are different in shape. In the b direction, the same type ofeight-ring channel is repeated, leading to a repeat unit in this direction of 14.3715
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(5) A, while in the c direction the two types of channel alternate, leading to anapproximate doubling of the unit cell dimension in this direction to 28.599 (1) A.
The overall structure of SIZ-6 is also shown in Figure 3.1. This is a very unusualmaterial comprising 13.5-A thick anionic aluminophosphate layers of chemicalcomposition Al4(OH)(PO4)3(HPO4)(H2PO4)−. The layers themselves consist ofrings containing four, six, and eight nodes (aluminum or phosphorus atoms).The eight-ring windows are large enough to make the layers potentially porous tosmall molecules. The layers are held together via some relatively strong hydrogenbonding. This occurs because two H2PO4 groups, one each from two adjacentlayers, forming dimeric units with O–O distances across the hydrogen bond of2.441 A. In addition, the negative charge on the layers is compensated for by one1-ethyl-3-methylimidazolium (EMIM) cation, which occupies the interlayer space.
3.4Ionothermal Synthesis of Silica-Based Zeolites
The ionothermal synthesis of AlPOs is relatively straightforward. Silicon-basedzeolites have, however, been much more of a challenge for ionothermal synthesis,although there has been more success in the synthesis of mesostructured silicausing ILs [37]. The problem with zeolite synthesis is primarily the solubility ofsilica starting materials in the commonly used ILs, which is not sufficiently good toallow silicate and aluminosilicate materials to be prepared. Before 2009, there wasonly one report of a silica polymorph being prepared from an IL [38] and one reportof the synthesis of a sodalite [39]. Successful synthesis of zeolites requires thepreparation of ILs more suited to silicate dissolution. Recently, in our laboratory,we were successful in preparing ILs comprising mixed halide and hydroxide anionsthat are suitable solvents for the preparation of purely siliceous and aluminosilicatezeolites. The presence of hydroxide increases the solubility of the silicate startingmaterials and allows the zeolites to crystallize on a suitable timescale (Figure 3.2,Wheatley and Morris, manuscript in preparation). However, despite this proof ofconcept work, there is still much to be done to more fully understand the chemistryof silica in ILs, and it is likely that task-specific ILs will need to be developed beforesilica zeolites can be prepared routinely using ionothermal synthesis.
3.5Ionothermal Synthesis of Metal Organic Frameworks and Coordination Polymers
Similar to the synthesis of zeolites, ILs can be used as solvents and templates toprepare many other types of solids. One of the most interesting and important classof materials that has been recently developed is that of metal organic frameworks(also known as coordination polymers) [40, 41]. These materials offer great promisefor many different applications, particularly in gas storage [42–45]. Normally thesematerials are prepared using solvothermal reactions, with organic solvents such as
3.6 Ambient Pressure Ionothermal Synthesis 93
TON MFI
EMIM Br/OH mixed IL
a
c
Figure 3.2 The ionothermal synthesis of pure silica zeolites(TON and MFI) using 1-butyl, 3-methyl imidazolium–basedionic liquids with mixed bromide-hydroxide counteranions.The BMIM cation can be clearly seen from the single-crystalX-ray diffraction structure of MFI.
alcohols and dimethyl formamide. Ionothermal synthesis has been used extensivelyover the last few years to prepare these types of solid, and there are now manyexamples in the literature [46–55].
Unlike zeolites, however, the lower thermal stability of coordination polymersleads to several issues regarding removal of ionic templates from the materialsto leave porous materials. Often removing the IL cation is not possible withoutcollapsing the structure. However, it is possible to prepare porous materials usingDESs, and Bu has recently proven this very elegantly [56].
A great many of the materials prepared ionothermally are relatively low-dimensional solids, and this is clearly a very productive method for the prepa-ration of such materials. It is very clear that in these systems changing thechemistry of the solvent to ionothermal leads to great possibilities in this area.
3.6Ambient Pressure Ionothermal Synthesis
Perhaps the most striking feature of ILs is their very low vapor pressure. This meansthat, unlike molecular solvents such as water, the ILs can be heated to relatively hightemperatures without the production of autogenous pressure. High-temperaturereactions therefore do not have to be completed inside pressure vessels such asTeflon-lined steel autoclaves but can be undertaken in simple containers such asround-bottomed flasks. The absence of autogenous pressure at high temperaturealso makes microwave heating a safer prospect as hot spots in the liquid should
94 3 Ionothermal Synthesis of Zeolites and Other Porous Materials
not cause excessive increases in pressure with their associated risk of explosion,assuming of course that the IL is stable and does not breakdown into smallercomponents during heating [57, 58]. Figure 3.3 shows the measured pressureduring the synthesis of an aluminophosphate molecular sieve (SIZ-4) using amicrowave heating experiment [59]. Figure 3.3a is the pure IL solvent, and it isclear that no autogenous pressure is produced. Figure 3.3b, however, shows that,even when only modest amounts of water are added to the system, significantpressures are evolved.
One of the most interesting potential uses of ambient pressure synthesis ofzeolite coatings is for anticorrosion applications. Yushan Yan has shown thationothermally prepared zeolite films make excellent anticorrosion coatings forseveral different types of alloys [60, 61]. Given that current coatings technologyis based on the use of environmentally unfriendly chromium, there is interestin finding more acceptable alternatives. Sealed zeolites are one such option.However, hydrothermal synthesis of zeolites inside sealed vessels is impractical for
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0 20 40 60 80 100 120
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Time (min)
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Figure 3.3 The evolution of pressure (inbar) in the microwave synthesis of alu-minophosphate SIZ-4 from (a) a pure ionicliquid solvent with no water added and (b)the same solvent system with 0.018 ml of
water added. The maximum temperatureis 200 oC and the duration of heating is60 minutes. There is almost no pressureevolution in the pure ionic liquid.
3.7 The Role of Cation-Templating, Co-Templating, or No Templating 95
large, oddly shaped, and cut pieces of metal. Yan contends that ambient pressureionothermal synthesis eliminates the need for unwieldy sealed vessels, and, giventhe excellent coatings that can be prepared using this approach, offers an interestingand potentially important alternative technology.
3.7The Role of Cation-Templating, Co-Templating, or No Templating
The original concept behind ionothermal synthesis was to simplify the templatingprocess that occurs in traditional zeolite hydrothermal synthesis by making thesolvent and the template the same species. The template molecules normallyinvolved in zeolite synthesis are usually cationic as the resultant framework hasa negative charge. The commonly used templating cations are very similar inchemistry to IL cations. It is not surprising therefore that the IL cations are oftenoccluded into the final structures of the materials, in exactly the same way as intraditional zeolite synthesis [9].
In an exactly analogous fashion, metal organic frameworks can also be synthe-sized using the ILs as both the solvent and the template. Most solvothermallyprepared MOFs have neutral frameworks, but when the template is a cation theframework must, for charge balance, have a negatively charged framework, inexactly the same way as zeolites. Of course, the overall goal of all templating-basedsynthesis is to have control over the architecture of the final material by changingthe size of the templating cation. It is well known, however, that apart from roughcorrelations with the size of the cation, the templating interaction is not reallyspecific enough to yield very precise control over the reaction. Figure 3.4 shows thatthe same general features hold for ionothermal synthesis. In this work, changingthe size of the IL cation does have some effect on the final structure – the largercations form more open frameworks with the extra space needed to accommodatethe large template. However, this is not particularly specific in this type of MOFsynthesis, indicating that templating is more likely to be by simple ‘‘space filling’’rather than any more specific or directed template–framework interactions (Linand Morris, Unpublished work).
In hydrothermal synthesis, there is also the possibility of adding alternativecations to act as templates. Of course, the situation is exactly analogous inionothermal synthesis, and added templates offer equally great opportunities.Recently, Xing et al. [62] have shown that methylimidazolium (MIA), when addedto an EMIM Br IL leads to a cooperative templating effect, occluding both MIA andEMIM in the same solid. The intriguing feature of this solid is that it seems, atleast on first inspection, that the material is made of two distinct layers. The MIAis located close to one layer and the EMIM close to the other – perhaps indicatingthat each cation plays a specific role in directing the structure of each part of thematerial. It is, of course, impossible to say this for certain until the full mechanismof synthesis is elucidated, something that is very difficult in practice. However,further circumstantial evidence for this maybe the fact that the previously prepared
96 3 Ionothermal Synthesis of Zeolites and Other Porous Materials
N+Br
N+Br
N N+Br
N+Br
N N+Br
N N+Br
Figure 3.4 The effect of changing the size of the IL cationon the resulting metal organic framework structure. The ma-terials prepared in this study are Ni (blue) or cobalt (purple)terephthalate MOFs.
EMIM-templated materials SIZ-1 and SIZ-4 have closely related structures to theEMIM-‘‘templated’’ layer in this material.
Up to now, the cation in the IL has only acted as a template in the synthesis.However, like any other solvent, including water, there is also the possibilityof bonding interactions with the frameworks. Most of the ILs that are based ondi-alkylated imidazolium cations have no obvious sites through which to coordinateto the metal sites in the way that water does. However, some ILs, under specificconditions, can breakdown to leave the monoalkylated imidazole species that cancoordinate to metals [63]. As in hydrothermal synthesis, controlling how the solventinteracts with the framework materials is therefore important in determining theexact nature of the final material. A similar example where the solvent cancoordinate to the metal in a metal organic framework comes when using cholinechloride/urea-based DES ILs. Normally, this type of solvent is regarded as beingrelatively unstable, especially the urea portion which can break up and deliversmaller templates into the reaction. However, under conditions where the urea isstable, it is possible to keep this intact, and in the case of ionothermally preparedlanthanum-based MOFs the urea coordinates to the metal [64].
In addition to the templating cations, ILs also contain an anion, and theseturn out to be extremely important in controlling the properties of the solvents(Section 3.8). The anions can, in certain circumstances, also be occluded in thestructure as a template, most often in combination with the IL cation. Bu et al.recently showed that in a series of MOFs (called ALF-n) the IL displayed severaldifferent types of behavior, including templating by only the cation and templating
3.8 The Role of the Anion – Structure Induction 97
by both the cation and anion simultaneously, illustrating the multiple functionsthat ILs can play even in the same systems [56].
Finally, of course, the ILs can only play the role as solvent and not be occludedin the final structure at all. For species like aluminophosphate zeolites and MOFswhere the chemistry of the cation is similar to that of commonly used templatesone would expect them to be occluded in the final structure. However, there arecertain situations where this does not happen. Perhaps the most striking of theseis when a very hydrophobic IL is used. In the case of aluminophosphate and MOFsynthesis, the more hydrophobic the IL used the less likely the IL cation is to beoccluded [32]. Of course, as the chemistry of the system is changed (e.g., by tryingto make different types of inorganic material), the balance between the solvent andtemplating actions of ILs also changes.
3.8The Role of the Anion – Structure Induction
As we have seen above, the common organic cations in ILs are chemically verysimilar to zeolite templates. However, ILs also contain an anion, and the natureof the anion plays an extremely important part in controlling the nature of the IL.Figure 3.5 demonstrates this dependence of property on anion very clearly. Twolow melting ILs that are solid at room temperature can be prepared from the samecation (EMIM) but with two different anions – bromide and triflimide (NTf2). Thetwo ILs have very different properties, especially when it comes to their interactionwith water. Figure 3.6 shows what happens when the two compounds are left outin the air for 20 minutes. EMIM NTf2 is a relatively hydrophobic material and thereis no change in its properties on exposure to the moisture in the air. EMIM Br, onthe other hand, is highly hygroscopic and turns liquid on reaction with moisturein the air.
Clearly, this change in IL chemistry on alteration of the IL anion is boundto have a significant effect on the products of any reaction carried out in suchsolvents. One example of this is given in Section 3.7, where in the synthesis ofaluminophosphates EMIM Br solvents lead to incorporation of the EMIM cationto form zeotype materials, whereas the use of the EMIM NTf2 IL leads to noocclusion of the IL cation [32]. More interesting, however, and potentially extremelyuseful, is the possibility of mixing the two types of liquid to form solvents withdifferent chemistries from the end member liquids. Figure 3.6 illustrates this forthe synthesis of cobalt bezenetricarboxylate MOFs [65]. The two end member ILs,EMIM Br and EMIM NTf2, form two different types of material, while a 50 : 50mixture of the two ILs, which are miscible, forms a third structure type. This typeof result opens up the possibility of mixing ILs to form solvents whose chemistryis different from the end members, giving rise to much more control over theproperties of the solvent. In a similar example, a mixed anion IL (50% bromide 50%triflimide) leads to the formation of coordination polymers containing fluorinated
98 3 Ionothermal Synthesis of Zeolites and Other Porous Materials
A B
A B
Figure 3.5 The effect of moist air on hydrophobic EMIMtriflimide (sample A) and hydrophilic EMIM Br (sample B).After 20 minutes exposure to normal air at room tempera-ture, the EMIM Br has absorbed enough moisture from theatmosphere to turn from a solid into a liquid.
ligands when ILs containing only one anion (either bromide or triflimide) does notproduce any crystalline solid [66].
It is clearly the nature of the anion that determines the final material in theseexamples. However, the anions themselves are not generally occluded into thestructure, and so this is an induction effect rather than a templating of the structuredirecting effect. It is perhaps not too surprising that changing the chemistry of thesolvent will change the type of product in such a manner. In the example illustratedin Figure 3.6, there is no obvious correspondence between the nature of the anionsand the nature of the final material. However, in 2007, we published an exampleof an anion induction using a chiral anion as part of an IL to induce a chiralcoordination polymer that contains only achiral building blocks [67] (Figure 3.7).In this example, a chiral IL prepared from the butyl methyl imidazolium (BMIM)cation in combination with l-aspartate as the anion, when used to prepare a cobaltbezenetricarboxylate MOF produced a chiral structure, with all indications that thebulk solid produced was homochiral. Where some specific property of the IL anionmanifests itself in the resulting material, despite the fact that it is not actuallyoccluded, the potential for ‘‘designer’’ structure induction becomes very attractive,and one would hope that such properties of ionothermal synthesis will be exploredand exploited more thoroughly in the near future.
3.9 The Role of Water and Other Mineralizers 99
100% bromide
50% bromide 50% triflimide
100% triflimide
N N
Br+
Figure 3.6 The effect of the anion on thefinal structure of the material produced inan ionothermal synthesis. The top reactionshows uses EMIM Br as the solvent, andproduces one particular cobalt-trimesic acid
MOF. A 50 : 50 mixture of EMIM Br andEMIM triflimide produces a different MOF,while using only the EMIM triflimide pro-duces yet another material.
3.9The Role of Water and Other Mineralizers
One of the very first questions asked about ionothermal synthesis was whether theILs used were sufficient in their own right to promote the synthesis of zeolitesand other inorganic materials, particularly those oxides where water might catalyzethe condensation reactions needed to form the required bonds. One of the firstthings noted about ionothermal synthesis was that too much water was detrimentalto the formation of zeolites. At low concentrations of water, zeolites were themain products, but as more water was added to the IL solvents so that they wereabout equimolar in concentration only dense phases could be prepared. Wraggand coworkers studied this effect in more detail and confirmed through severalhundred high-throughput reactions that larger amounts of water did indeed leadto dense phases [63]. The origin of this effect is still under investigation but itis known that the microstructure of water in ILs changes with concentration.At low concentrations, the water is hydrogen bonded relatively strongly to theanion, and exists either as isolated water molecules or as very small clusters [68].However, as the concentration of water increases, larger clusters and eventually
100 3 Ionothermal Synthesis of Zeolites and Other Porous Materials
N N+ HO
O
O
O
NH2
−
Figure 3.7 The use of an ionic liquid with a chiral anioninduces a chiral MOF structure. Use of an achiral anionproduces achiral structures.
hydrogen-bonded networks start to appear, which change the properties of theliquid markedly. Eventually, of course, as more and more water is added, itbecomes the dominant chemical component (and therefore the solvent) and thesystem becomes hydrothermal rather than ionothermal.
The strong binding of isolated water molecules in ILs leads to another interestingeffect that can be used in ionothermal synthesis – so-called water deactivation. Atlow concentrations of water, this strong hydrogen bonding leads to water beingless reactive than similar amounts in other solvents. This effect is so strongthat highly hydrolytically sensitive compounds such as PCl3 can be stored forrelatively long periods, whereas they react quickly, and often violently, in other‘‘wet’’ solvents [69]. Such water deactivation is probably the reason why some ofthe materials prepared using ILs can have unusual features. For instance SIZ-13, acobalt aluminophosphate material, has a layered structure that is closely related to azeolite, but has Co–Cl bonds. Normally such bonds are hydrolytically unstable and,under hydrothermal conditions, it is unlikely that this material would be stable [27].
In zeolite (and other) synthetic procedures, mineralizers, such as fluoride orhydroxide ions, added to the reaction mixtures in the correct quantities are oftenvital for crystallization of the desired molecular sieve products. Fluoride in particularhas recently been an extremely useful mineralizer for aluminophosphate [70] andsilicate [71, 72] synthesis. In addition to helping solubilize the starting materialsunder the reaction conditions, there is evidence that fluoride itself can play astructure directing role [73] and is intimately involved in template ordering incertain materials [74, 75]. In ionothermal synthesis, the addition of fluoride alsoseems to be important in determining the phase selectivity of the reaction [9]. It mayalso help catalyze the bond-forming reactions in zeolite synthesis, as suggested byCamblor and coworkers [71]. For instance, in the synthesis of aluminophosphates,
3.11 Summary and Outlook 101
the addition of fluoride leads to the formation of SIZ-4 and SIZ-3, which are bothfully four-connected zeolite frameworks, SIZ-1, which is an interrupted structurewith some unconnected P–OH bonds.
Tian and coworkers have recently completed an extremely useful kinetic studyof the effect of both water and fluoride added to ionothermal systems in zeolitesynthesis [76]. It is clear from their results that both small amounts of water, andparticularly fluoride, increase the crystallization rate. If the reactions are carriedout carefully to exclude as much water as possible, the crystallization of the zeolitesbecomes very slow indeed, suggesting that for all practical purposes a smallreactant amount of water (probably in the IL) is vital if ionothermal synthesis is tobe successful.
3.10Unstable Ionic Liquids
In many publications, one often sees the mention of the high thermal and chemicalstability of ILs. Bearing in mind of course that it is difficult to generalize across allthe possible ILs, this is true under many conditions. However, under ionothermalconditions, some quite common ILs can breakdown. Even some that are oftenrelatively stable such as BMIM bromide can breakdown, especially in the presenceof fluoride ions [77]. One possible reaction is the transalkylation reaction that swapsthe alkyl groups, leading to the formation of dimethylimidazolium cations, whichthen templates a zeolite structure [77].
DES ILs based on choline chloride/urea mixtures are also unstable underionothermal conditions. The urea portion of the IL breaks up to release ammoniumions into the mixture, which then templates the SIZ-2 aluminophosphate material.This type of instability in the ILs is actually extremely repeatable. Deep eutecticILs made from functionized ureas all break down in the same way to produce theexpected functionalized ammonium or diammonium cations that then go on totemplate many different structures [78]. Such reproduction ability in the reactions ofthese ILs opens up interesting possibilities for the delivery of small amounts of tem-plate to the reaction mixture, as opposed to having the whole IL made up of thetemplate.
3.11Summary and Outlook
Normally, ILs are classed as ‘‘green’’ chemicals because they are most often usedto replace volatile organic solvents. However, when preparing the materials, thisperspective has been discussed and, in particular, all inorganic framework solidssuch as zeolites and ILs are more often than not replacing water. In these situations,ionothermal synthesis cannot be called a green technology compared to that whichit replaces. When replacing organic solvents in, for example, the synthesis ofmetal organic frameworks there is more justification for using the ‘‘green’’ tag.
102 3 Ionothermal Synthesis of Zeolites and Other Porous Materials
However, even in these syntheses, the success of the methodology has to relyon the ILs introducing new chemistry into the system that is not possible usingother systems. Fortunately, over recent years, ionothermal synthesis has beenrecognized as a highly flexible methodology that does indeed bring new chemistryto the system. Features like water deactivation and chiral induction offer manypossibilities for the preparation of materials that are unlikely or even impossible tomake in other solvents.
One of the most interesting features of ILs for ionothermal synthesis is the sheernumber of possible liquids available. There are an estimated 1 million binary ILsavailable, compared to only a few hundred molecular solvents. The wide range ofaccessible properties of the liquids provides huge opportunities for matching thechemistry of the solvent system to that of the reactants. However, this also presentshuge challenges – it is at the moment extremely difficult to predict a priori theproperties of the solvent and how they will behave in combination with the reactants.Up to now, only a few of the easily available ILs have been studied, leaving manypotentially interesting solvents completely unexplored. One particularly interestingfeature of ionothermal synthesis is the use of mixed ILs to tailor the solvent toward aparticular reaction chemistry by mixing two different miscible ILs to produce a newsolvent with different properties (Section 3.8). Once again the issue of predictingthe properties of the mixed ILs is a problem. However, this type of approach isparticularly suited to high-throughput methodologies because new solvents canbe prepared simply by mixing two ILs in various amounts, and the ‘‘brute force’’approach afforded by high-throughput instrumentation can at least identify areasof interest in the compositional fields.
The use of ILs in the synthesis of solids has, of course, not been limited tonew hybrid and inorganic framework solids. Work in the nanomaterials area andincreasingly in other areas, such as the organic solid state, has increased steadilyover the last few years. However, there is still much scope to develop the synthesismethodology further.
In the field of zeolite science, the challenges are clear, particularly for thesynthesis of silica-based zeolites. Here the plethora of possible ILs is both ablessing and a challenge as we really need to understand more fully the speciationof silicate ions in particular when they are dissolved in ILs. It is clear that thechange from molecular to ionic solvents significantly affects the chemistry, andthat new zeolite-type structures will inevitably arise from ionothermal preparations.We hope that as we discover ever more about the interesting properties of ILs thefield of ionothermal synthesis will develop into an even more useful addition to thearmory of synthetic zeolite chemists.
References
1. Davis, M.E. (2002) Nature, 417,813–821.
2. Paillaud, J.L., Harbuzaru, B., Patarin,J., and Bats, N. (2004) Science, 304,990–992.
